Commercial, or civil, aircraft are conventionally designed for reducing exhaust emissions from combustion of hydrocarbon fuel such as, for example, Jet-A fuel. The exhaust emissions may include hydrocarbon particulate matter, in the form of smoke for example, carbon monoxide, and nitrogen oxides (NO.sub.x) such as, for example nitrogen dioxide NO.sub.2. NO.sub.x emissions are known to occur from combustion at relatively high temperatures, for example over 3,000.degree. F. (1648.degree. C.). These temperatures occur when fuel is burned at fuel/air ratios at or near stoichiometric, i.e. at an equivalence ratio of 1.0 which represents the actual fuel-air ratio divided by the stoichiometric fuel/air ratio. The amount of emissions formed is directly related to the time that combustion takes place at these conditions, i.e., residence time.
Conventional gas turbine engine combustors for use in an engine for powering an aircraft are conventionally sized and configured for obtaining varying fuel equivalence ratios during the varying power output requirements of the engine such as, for example, lightoff, idle, takeoff, and cruise modes of operation of the engine in the aircraft. At relatively low power modes, such as at lightoff and idle, a relatively rich fuel/air mixture with an equivalence ratio greater than one is desired for initiating combustion and maintaining stability of the combustion. At relatively high power modes, such as for example cruise and takeoff operations of the engine in the aircraft, a relatively lean fuel/air mixture with an equivalence ratio less then one is desired for obtaining reduced exhaust emissions.
In the cruise mode, for example, where an aircraft gas turbine engine operates for a substantial amount of time, conventional combustors are typically sized for obtaining combustion at generally stoichiometric fuel/air ratios, equivalence ratio of 1.0, in the dome region, which represents theoretically complete combustion. However, in practical applications, exhaust emissions nevertheless occur, and conventional combustors utilize various means for reducing exhaust emissions.
As government exhaust emissions regulations become stricter, most notably, those regulations relating to NO.sub.x emissions, gas turbine engine combustors will require improved means for reducing such emissions. This is true for, most significantly, gas turbine engine powered commercial aircraft, as well as military aircraft and marine and industrial (M&I) gas turbine engines. Furthermore, the trend in present gas turbine engine design is to decrease engine, and combustor, length for reducing weight and parasitic cooling air requirements for increasing engine efficiency and specific fuel consumption among other things. However, combustion gases discharged from a combustor must have relatively uniform temperature distribution for obtaining acceptable life of turbine nozzle vanes and rotor blades disposed downstream therefrom. Accordingly, combustor length reduction is limited by the ability to effectively mix the combustion gases therein for obtaining relatively uniform combustion gas exit temperature.